This invention is generally directed to processes for the preparation of polycarbonates, inclusive of those obtained from biphenols such as gem-bis(hydroxyaryl)alkanes, reference U.S. Pat No. 4,766,255, entitled Processes for Bisphenols, the disclosure of which is totally incorporated herein by reference. More specifically, the present invention relates to processes for the purification of polycarbonates, especially polycarbonate (A), polycarbonate (Z), copolymers thereof, and the like, wherein undesirable impurities such as catalysts selected for the preparation thereof are removed. In one embodiment of the present invention there is provided a process for the purification of polycarbonates obtained from, for example, the known melt polyesterification reactions, which process comprises the formation of a complex with a titanium catalyst, and the removal thereof enabling polycarbonates with improved characteristics. Accordingly, with the process of the present invention there are obtained polycarbonates substantially free of titanium butoxide catalysts, which catalysts are selected in some instances for the preparation of polycarbonates. The purified polycarbonates obtained with the process of the present invention possess improved characteristics, including low dark decay values, and minimum residual potential with cycle up when these polycarbonates are selected as binders for transport molecules in layered imaging members as illustrated, for example, in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference. Also, with the purified polycarbonates of the present invention the undesirable crystallization of aryl amine molecules, which are dispersed therein for the formation of the aforementioned imaging members, is eliminated. Additionally, the toxic chemical phosgene is avoided with the process of the present invention. Also, with the process of the present invention there can be obtained purified polycarbonates of a weight average molecular weight of from about 25,000 to about 100,000, and preferably from about 26,000 to about 55,000. These molecular weights result, it is believed, since the more active catalysts comprised of titanium or zirconium alkoxides are initially selected for the preparation of the polycarbonate, which is then purified in accordance with the process of the present invention. When less active manganese acetate is selected for the preparation of polycarbonates, the weight average molecular weight of the polycarbonates obtained is lower, that is less than about 20,000 and about 16,000. The higher molecular weight polycarbonates of about 25,000 to about 100,000 possess increased toughness, and provide for a longer lasting and more resilient device when employed as the resin binder in layered imaging members.
Processes for the preparation of bisphenols are known, and generally involve the condensation of two mole equivalents of phenol with one more equivalent of carbonyl compound in the presence of an acid catalyst. Acid catalysts employed for the aforementioned condensation are concentrated hydrochloric acid, gaseous hydrogen chloride, concentrated sulfuric acid, hydrogen fluoride, hydrogen bromide, boron trifluoride, boric acid, ferric chloride, phosphorus chloride, phosphorus pentoxide, benzenesulfonic acid, and the like. Although these acid catalysts, in particular gaseous hydrogen chloride, are very effective in promoting the condensation of phenols with sterically accessible ketones such as acetone, they are not effective for the preparation of bisphenols, especially bisphenol (Z) derived from the sterically demanding small cyclic ketone such as cyclopentanone or cyclohexanone. The condensation with small ring ketones does not normally proceed in a rapid manner, and the yield of product is generally less than desirable. The aforementioned reaction, especially when accomplished in the presence of hydrogen chloride as a catalyst, is illustrated in U.S. Pat. No. 4,304,899. Similar teachings are presented in U.S. Pat. Nos. 1,760,758; 2,069,560 and 2,069,573, wherein there are disclosed methods for the preparation of bisphenols with hydrogen chloride catalysts. The polycarbonates resulting from these bisphenols can be selected as resinuous binders for aryl amine hole transport compounds.
In U.S. Pat. No. 2,858,342, there is disclosed, for example, a method for the preparation of bisphenols utilizing alkali metal phenoxides, or alkaline earth metal phenoxides of the phenol being reacted; and wherein cyclohexanone may be selected as a reactant. There resulted in one process embodiment illustrated in this patent, reference Example XI, the preparation of 1,1-bis-(4-hydroxyphenyl)cyclohexane. Also, there is described in U.S. Pat. No. 4,423,352 a process for the preparation of bisphenols utilizing a cation exchange resin modified with a pyridine alkanethiol as a catalyst.
Moreover, U.S. Pat. No. 1,977,627 describes a process for the preparation of bisphenols wherein 65 to 75 percent sulfuric acid is selected as the catalyst. With the process as disclosed in the '627 patent, there is avoided a complex apparatus, and moreover corrosion problems are substantially reduced. In comparison to the processes mentioned herein, wherein, for example, hydrogen chloride is selected as a catalyst, the process of the '627 patent proceeds in a less rapid manner and the product resulting is more difficult to purify. Additionally, it is known that certain sulfur compounds such as sulfur dichloride, sodium thiosulfate, sodium sulfide and the like can be selected for the synthesis of bisphenols, reference for example U.S. Pat. No. 2,923,744, which illustrates a process for the preparation of bisphenols wherein there is selected mercaptoalkanesulfonic acids in catalytic amounts for the purpose of promoting the condensation of phenols and carbonyl compounds. Similarly, selenium and tellurium compounds are effective catalysts for bisphenol synthesis, reference for example U.S. Pat. No. 2,762,846.
Furthermore, in the aforesaid U.S. Pat. No. 4,766,255 there is illustrated the preparation of bisphenols, which comprises the reaction of ketones and phenols in the presence of halotrialkylsilanes and a thiol catalyst. In one specific embodiment of the copending application, bisphenols are obtained by reacting ketones with an excess of phenols in the presence of a stoichiometric quantity of chlorotrimethyl silane and a catalytic amount of, for example, an alkanethiol.
Accordingly, in one embodiment of the aforementioned copending application there is illustrated the preparation of polycarbonates from bisphenol (Z), which is formed by the reaction of cyclohexane, and an excess amount of phenol in the presence of a stoichiometric quantity of chlorotrimethyl silane and a catalytic amount of butanethiol, which reaction is acomplished at at temperature of from about 30.degree. to about 65.degree. C. and is illustrated with reference to the following illustrative reaction scheme ##STR1##
Illustrative examples of ketones usually selected in one mole equivalent that may be utilized for the aforementioned process include acetones, butanones, pentanones, cyclopentanones, substituted cyclopentanones, cyclohexanones, substituted cyclohexanones, and the like. As hydroxyarenes, such as phenols, present in an amount of from about 2 to 10 mole equivalents, there can be selected for this process of the present invention phenols, cresols, ethylphenols, halophenols, cyanophenols, nitrophenols, naphthols, and the like. Examples of alkylhalosilanes present in an amount of from about 0.1 to 3 mole equivalents that can be selected for this process include chlorotrimethyl silane, dichlorodimethyl silane, methyltrichloro silane, bromotrimethyl silane, fluorotrimethyl silane, chlorotriethyl silane, bromotriethyl silane, fluorotriethyl silane, and other similar silanes wherein the alkyl substituent contains, for example, from 1 to about 10 carbon atoms; and the halogen substituent can be fluoro, chloro, bromo, or iodo.
The bisphenols obtained with the processes of the aforesaid copending application can be selected for the preparation of resin binder polycarbonates by the reaction thereof with carbonate precursors such as phosgene, diacyl halides, bishaloformates, diesters, and diarylcarbonates. Polycondensation of bisphenols with phosgene, diacyl halide and bishaloformate can be executed in a suitable medium such as methylene chloride in the presence of a base such as pyridine. Also, the polycondensation reaction can also be conveniently accomplished by interfacial polymerization. The polycondensation of a bisphenol with a diester or diarylcarbonate requires an efficient catalyst such as titanium alkoxides, high temperatures, and high vacuum with an efficient condenser to remove the displacement byproduct. More specifically, polycarbonates such as PC(A) and PC(Z), for instance, are respectively prepared by reacting at room temperature stoichiometric quantities of bisphenol (A) and bisphenol (Z) with phosgene in methylene chloride in the presence of pyridine. These polycarbonates can also be prepared by interfacial polymerization of bisphenols with phosgene in a water methylene chloride medium containing a suitable water soluble base. Further, the aforementioned polycarbonates can be synthesized by transesterification with diphenylcarbonates in the presence of a catalyst such as titanium isopropoxide at high temperatures under high vacuum conditions. In the latter process, an efficient stirring mechanism and a condensing system to remove the phenol byproduct during the course of the polymerization can be utilized.
Polycarbonates with a weight average molecular weight, Mw, of from about 20,000 to about 200,000, and preferably from about 25,000 to about 100,000 prepared from the aforementioned bisphenol are useful as resinous binders for photogenerating pigments and charge transport molecules in layered photoresponsive imaging members as illustrated, for example, in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference. More specifically, these members are usually comprised of a supporting substrate, a photogenerating layer containing there from about 85 to about 99 percent by weight of trigonal selenium, metal phthalocyanines, or metal free phthalocyanines, dispersed in the polycarbonate resinous binder present in an amount of from about 1 to about 15 percent by weight; and a charge transport layer comprised of, for example, aryl amines of the formula as illustrated in the '990 patent, and copending application U.S. Ser. No. 851,051 (now abandoned) relating to organic photoconductive imaging members, the disclosure of which is totally incorporated herein by reference. The aforementioned photoconductive imaging members can be incorporated into numerous imaging and printing processes and apparatuses inclusive of xerographic imaging methods.
Kosji Ueno and S. Hasegawa, Japanese Patent Application 1971-61,342, disclose a process for the preparation of polycarbonate Z products using metallic acetates, principally manganese acetates. Although these products have acceptable xerographic properties following precipitation from toluene into methanol, manganese acetate catalyzed polymerizations do not enable high molecular weight polymers as indicated herein. With high molecular weight polycarbonates, from about 25,000 to about 100,000 as indicated herein, there is provided a tougher material for the photoreceptor. Also, numerous processes are known for deactivating titanium alkonate catalyst. While these processes inhibit the catalytic action of the titanium compounds selected, they do not eliminate the deleterious influence of the catalyst residue on the xerographic performance of the polymer.
There is disclosed in U.S. Pat. No. 3,483,157 processes utilizing arsenic compounds or steam. Specifically, the processes illustrated in the '157 patent were designed for catalyst deactivation so that further processing steps do not trigger additional polymerization, particularly in the blending steps hence they are catalyst "poisons" and are not removed from the polymer since, for example, the catalyst is not soluble in water. Also, arsenic compounds are extremely toxic. Moreover, in French 2,343,778 (1977) and Miller German Offen. DE 2506353, 1975 there is disclosed phosphorus complexation for the purpose of catalyst deactivation, however, both of these methods retain molecular species in the polymer that will negatively affect its xerographic performance. The fiber industry's practice of treating titanium catalyzed polymer with aqueous base produces a colorless polymer, however, the now colorless titanium residue imparts poor xerographic performance to the polymer.
Accordingly, while processes for the preparation of polycarbonates are known, there is a need for processes for the purification thereof. More specifically, there is a need for processes that will enable the removal of undesirable impurities from the polycarbonates obtained. Additionally, there is a need for purification polycarbonate processes wherein titanium catalysts are removed thereby permitting layered imaging members containing the polycarbonates to possess improved electrical characteristics, including low dark decay values. Furthermore, there is a need for processes for the purification of polycarbonates, which are substantially free of titanium impurities, enabling their use, for example, as binders in electrophotographic imaging members, and wherein the charge transport molecules selected are substantially free of crystallization. In addition, there is a need for economical processes that enable the purification of polycarbonates wherein titanium catalysts selected for the preparation thereof are substantially removed. Also, there is a need for efficient processes that permit the purification of polycarbonates that employ reagents of low or no flammability. There is also a need for processes that produce colorless polymers from titanium catalyzed polymerizations.